1. Field of the Invention
The present invention relates to an in-plane switching (IPS) mode liquid crystal display (LCD) device and more particularly to an array substrate for an IPS mode LCD device being capable of preventing a color shift problem and increasing an aperture ratio.
2. Discussion of the Related Art
A related art liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite alignment direction as a result of their thin and long shapes. The alignment direction of the liquid crystal molecules can be controlled by applying an electric field across the liquid crystal molecules. In other words, as the intensity or direction of the electric field is changed, the alignment of the liquid crystal molecules also changes. Since incident light is refracted based on the orientation of the liquid crystal molecules due to the optical anisotropy of the liquid crystal molecules, images can be displayed by controlling light transmissivity.
Since the LCD device including a thin film transistor (TFT) as a switching element, referred to as an active matrix LCD (AM-LCD) device, has excellent characteristics of high resolution and displaying moving images, the AM-LCD device has been widely used.
The AM-LCD device includes an array substrate, a color filter substrate and a liquid crystal layer interposed therebetween. The array substrate may include a pixel electrode and the TFT, and the color filter substrate may include a color filter layer and a common electrode. The AM-LCD device is driven by an electric field between the pixel electrode and the common electrode to have excellent properties of transmittance and aperture ratio. However, since the AM-LCD device uses a vertical electric field, the AM-LCD device has a bad viewing angle.
An in-plane switching (IPS) mode LCD device may be used to resolve the above-mentioned limitations. FIG. 1 is a cross-sectional view of an IPS mode LCD device according to the related art. As shown in FIG. 1, the array substrate and the color filter substrate are separated and face each other. The array substrate includes a first substrate 10, a common electrode 17 and a pixel electrode 30. Though not shown, the array substrate may include a TFT, a gate line, a data line, and so on. The color filter substrate includes a second substrate 9, a color filter layer (not shown), and so on. A liquid crystal layer 11 is interposed between the first substrate 10 and the second substrate 9. Since the common electrode 17 and the pixel electrode 30 are formed on the first substrate 10 on the same level, a horizontal electric field “L” is generated between the common and pixel electrodes 17 and 30. The liquid crystal molecules of the liquid crystal layer 11 are driven by a horizontal electric field such that the IPS mode LCD device has a wide viewing angle.
FIGS. 2A and 2B are cross-sectional views showing turned on/off conditions of an IPS mode LCD device according to the related art. As shown in FIG. 2A, when the voltage is applied to the IPS mode LCD device, liquid crystal molecules 11a above the common electrode 17 and the pixel electrode 30 are unchanged. But, liquid crystal molecules 11b between the common electrode 17 and the pixel electrode 30 are horizontally arranged due to the horizontal electric field “L”. Since the liquid crystal molecules are arranged by the horizontal electric field, the IPS mode LCD device has a characteristic of a wide viewing angle. FIG. 2B shows a condition when the voltage is not applied to the IPS mode LCD device. Because an electric field is not generated between the common and pixel electrodes 17 and 30, the arrangement of liquid crystal molecules 11 is not changed.
FIG. 3 is a plane-view showing one pixel region of an array substrate for an IPS mode LCD device according to the related art.
As shown in FIG. 3, a gate line 43, a common line 47, which is parallel to and spaced apart from the gate line 43, a data line 60, which crosses the gate line 43 to define a pixel region “P”, are formed on a substrate 40.
A thin film transistor (TFT) “Tr” is formed at a crossing portion of the gate and data lines 43 and 60. The TFT “Tr” includes a gate electrode 45, a semiconductor layer (not shown), a source electrode 53 and a drain electrode 55. The source electrode 53 and the gate electrode 45 respectively extend from the data line 60 and the gate line 53 such that the TFT “Tr” is connected to the data line 60 and the gate line 43.
In addition, a plurality of pixel electrodes 70a and 70b, which are electrically connected to the drain electrode 55 through a drain contact hole 67, and a plurality of common electrodes 49a and 49b are formed in the pixel region “P”. The common electrodes 49a and 49b are alternately arranged with the pixel electrodes 70a and 70b and extend from the common line 47.
Unfortunately, since a single domain is generated in one pixel region, there are color shift problems at upper-right, upper-left, lower-right and lower-left sides. Particularly, a yellow color shift problem is strongly generated at the upper-left side, i.e., 10 o'clock direction, and a blue color shift problem is strongly generated at the upper-right side, i.e., 2 o'clock direction.
To resolve the above color shift problems, an array substrate, where a center of each of the pixel and common electrodes is bent such that the array substrate has a double-domain structure, is introduced.
FIGS. 4A and 4B are schematic views for illustrating an array substrate having a double-domain structure. FIGS. 4A and 4B show a common electrode 80, a pixel electrode 83, a rubbing direction “rb”, a first polarizing axis “POL1” and a second polarizing axis “POL2” of polarizing plates, and a director of liquid crystal molecules 90 at a low gray level. The common and pixel electrodes 80 and 83 are symmetrically bent at a center such that a double-domain structure is generated at one pixel region “P”. As a result, a color shift problem is prevented by a counterbalance of domains.
In more detail, when the rubbing direction “rb” is parallel to one of the first and second polarizing axis “POL1” and “POL2”, which are perpendicular to each other, a director of the liquid crystal molecule 90 at the first domain “D1” and a director of the liquid crystal molecule 90 at the second domain “D2” are perfectly symmetric to each other, as shown FIG. 4B showing a schematic view of a full white mode. As a result, a color shift problem is prevented.
However, as shown in FIG. 4A showing a schematic view of a low gray level mode, a director of the liquid crystal molecule 90 at a first domain “D1” and a director of the liquid crystal molecule 90 at a second domain “D2” is imperfectly symmetric to each other such that a counterbalance of the domains “D1” and “D2” is also imperfect. As a result, a color shift problem is still generated.
In addition, since a boundary of the first and second domains “D1” and “D2” is positioned in the pixel region “P”, a light leakage problem is generated. When a light-shielding element, e.g., a black matrix, is formed to prevent the light leakage problem, an aperture ratio is decreased.